The high switching rates used in a modern integrated circuit may cause the supply voltage to degrade at certain points in the circuit and may cause circuit failures. Capacitors may be used to reduce supply voltage variations arising from high switching rates in the supplied circuits. These decoupling capacitors are connected between the supply voltage and ground in parallel with the supplied circuit. This parallel capacitance tends to decouple the voltage supply from disturbances induced by activity in the supplied circuit and allow the voltage supply to remain at the intended level.
In earlier integrated circuits, the decoupling capacitance could be placed off-chip due to the relatively slow cycle times at which the earlier circuits operated. The relatively low frequency response exhibited by these off-chip capacitor structures could still accommodate the relatively slow switching rates of the earlier circuits. In addition, the on-chip circuitry itself provided a large amount of near-by decoupling capacitance. As semiconductor fabrication technologies advance, however, circuit devices such as transistors are packed more and more densely on integrated circuit chips. At the same time, the resulting circuits operate at faster cycle times. At current cycle times on the order of 1 GHz, off-chip capacitance takes many processor cycles to respond. The slow frequency response of off-chip capacitance makes off-chip capacitor arrangements unsuitable for providing the decoupling capacitance necessary to prevent circuit failures in these faster integrated circuit devices. Further, silicon-on-insulator ("SOI") technology allows for still faster cycle times, while further reducing the capacitance of on-chip, non-switching circuitry.
In order to provide sufficient decoupling capacitance at the frequency response necessary in modern higher-frequency circuits, the capacitance must be moved closer to the switching circuitry, onto the integrated circuit chip itself. However, the on-chip capacitance must be provided in an area-efficient manner so as not to take up excessive space on the respective chip. Also, on-chip capacitors should be easy to fabricate along with the active and passive circuit elements which make up the desired integrated circuit.